1. Field of the Invention
The invention disclosed in this specification relates to a configuration of a pixel region of an active matrix type liquid crystal display and, more particularly, to a configuration of an auxiliary capacitor connected to a pixel electrode in parallel and a configuration of a black matrix for preventing leakage of light at boundaries between adjacent pixels.
2. Description of Related Art
Liquid crystal displays having an active matrix circuit are known. They have a configuration including a plurality of source bus lines for transmitting image data, a plurality of gate bus lines for transmitting switching signals in an intersecting relationship therewith, and a plurality of pixels provided at those intersections. Normally, transistors (specifically thin film transistors) are used as switching elements.
A pixel includes not only a transistor for switching but also a pixel electrode and has a structure in which the gate electrode, source, and drain of the transistor are connected to a gate bus line, a source bus line, and the pixel electrode, respectively. Distinction between the source and drain of a transistor is not fixed during the operation thereof and varies depending on signals according to a common definition of electrical circuits. In the following description, however, the terms xe2x80x9csourcexe2x80x9d and xe2x80x9cdrainxe2x80x9d simply refer to impurity regions provided in a transistor which are connected to a source bus line and a pixel electrode, respectively.
Each pixel includes one or more transistors. Specifically, two or more transistors connected in series are advantageous in that leak current can be reduced even when the transistors are not selected. The above-described definition is also applied to such a case, and no definition is given to an impurity region connected to neither a source bus line nor a pixel electrode.
A capacitor is formed between a pixel electrode and an electrode which is opposite to the pixel electrode across liquid crystal. A transistor as described above serves as a switching element for supplying and removing electrical charge to and from this capacitor.
In an actual operation, however, the capacity of the pixel electrode portion is too small to store necessary electrical charge for a sufficient period of time by itself. It is therefore necessary to provide a separate auxiliary capacitor.
Such an auxiliary capacitor (also referred to as xe2x80x9cstorage capacitorxe2x80x9d) has been formed between an opaque conductive material such as a metal which is separately provided and a pixel electrode or a semiconductor layer. The gate bus line for the next row has been normally used as the opposite electrode. Although a capacitor formed using a gate bus line has been sufficient when the area of the pixel is large enough, a gate bus line has been unsuccessful in providing a sufficient capacity by itself when the area of the pixel is small. This has required a gate bus line to be expanded to accommodate the area for an electrode of an auxiliary capacitor. Such a structure results in a decrease in the aperture ratio because a pixel includes an area where light is blocked.
It is an object of the invention disclosed in this specification to provide a configuration wherein an auxiliary capacity is increased without reducing the aperture ratio of a pixel. It is another object of the invention to provide a configuration of a black matrix for solving the problem of leakage of light that occurs at boundaries between adjoining pixels.
The invention disclosed in this specification is characterized in that:
a pixel includes a pixel electrode comprising a first transparent conductive film;
the pixel electrode overlaps a gate bus line and a source bus line;
a layer of a common electrode comprising a second transparent conductive film is provided between the gate bus line and the pixel electrode and between the source bus line and the pixel electrode such that it covers the gate bus line and source bus line; and
the common electrode is maintained at constant potential.
Specifically, the common electrode is disposed such that it covers the bus lines, and the pixel electrode is disposed such that it overlaps the bus lines. Thus, the common electrode and the pixel electrode overlap each other, and this overlap provides an auxiliary capacitor. In addition, while at least one of the electrodes of a conventional auxiliary capacitor has been formed of an opaque material, both of such electrodes are formed of a transparent material according to the present invention. Thus, they will not hinder display and the aperture ratio is maintained.
With the above-described configuration, a black matrix can be formed by source bus lines and gate bus lines by arranging pixel electrodes in an overlapping relationship with the source bus lines and gate bus lines and by positioning boundaries between the pixel electrodes on the source bus lines and gate bus lines. In general, boundaries between pixel electrodes of a liquid crystal display are affected by the electric field of the adjacent electrodes. As a result, at such boundaries, an image can be produced differently from that to be formed by the relevant pixels or leakage of light can occur due to absence of an electric field.
It is therefore not appropriate to use such boundaries between pixel electrodes, and a structure is normally employed in which such areas are shaded by a black matrix. It has been conventionally necessary to configure a black matrix by a separate layer. For example, the use of bus lines as a black matrix has been proposed in Japanese unexamined patent publication (KOKAI) No. H6-216421 and etc. In practice, however, this results in unstable display because signals on bus lines affect pixel electrodes.
The present invention solves this problem. Specifically, since a common electrode is provided between a bus line and a pixel electrode, a signal on the bus line is blocked by the common electrode and hence does not affect the pixel electrode.
When a top-gate type transistor (a transistor having a structure in which the gate electrode is provided on top of a semiconductor layer) is used as a switching transistor, it will be advantageous for stable operation of the transistor if light is allowed to be incident upon the substrate primarily from above, i.e., from the side of the pixel electrodes because this prevents light from entering the semiconductor layer under the gate electrode.
The effect of shading can be further improved for more stable operation by providing a film made of the same material as the layer of the source bus lines on top of the gate bus lines where the semiconductor layer and the gate bus lines intersect with each other.
As an insulator to be used between the common electrode and the pixel electrode, organic resin may be used as well as inorganic materials (e.g., silicon oxide and silicon nitride).
Especially, a flat insulation layer formed by means of spin coating or the like will be effective in reducing surface irregularity for improved uniformity of an electric field applied to liquid crystal molecules.
Materials usable for the transparent conductive film according to the invention disclosed in this specification include ITO (indium tin oxide), SnO2, and materials mainly composed of those materials as well.